The present application claims a priority under 35 U.S.C. xc2xa7119 to Japanese Patent Applications No. 2000-168903, filed on Jun. 6, 2000, entitled xe2x80x9cMETHOD FOR ESTIMATING QUALITY OF LEAD-FREE SOLDER MATERIAL AND PROCESS AND SYSTEM FOR FLOW SOLDERINGxe2x80x9d and No. 2000-168904, filed on Jun. 6, 2000, entitled xe2x80x9cAPPARATUS FOR ESTIMATING QUALITY OF LEAD-FREE SOLDER MATERIALxe2x80x9d. The contents of these applications are incorporated herein by the reference in their entirety.
1. Field of the Invention
The present invention relates to a method and an apparatus for estimating a quality of a lead-free solder material which is used for a flow soldering process of, for example, mounting an electronic component on a board (or a substrate). The present invention also relates to a process and a system for flow soldering which are combined with such method and/or apparatus for estimating the quality of the lead-free solder material.
2. Description of Related Art
For production of an electronic circuit board which is to be incorporated in an electronic device, electronic components are conventionally mounted on a board (or a substrate) such as a printed circuit board using a so-called xe2x80x9cflow solderingxe2x80x9d process (which is also called as a xe2x80x9cwave solderingxe2x80x9d process). A general flow soldering process will be described below.
Prior to soldering, there is prepared a board on which at least one electronic component is located by inserting a lead(s) (e.g. an electrode(s)) drawn from the electronic component into a through hole perforating through the board. It is noted that a land made of copper foil or the like is formed to cover a wall of the through hole as well as regions surrounding the through hole on an upside and an underside of the board, respectively, and this land is connected to a wiring pattern(s) on the upside of the board. In addition, regions on the upside and the underside of the board excluding the lands are coated with a solder resist to repel a molten solder material.
Next, while the board is mechanically conveyed at an approximately constant velocity in a flow soldering apparatus, a solder material which is preheated in a solder bath to melt is supplied in the form of a solder wave(s) to the board from the underside of the board. At this stage, the molten solder material which is supplied as described above rises from the underside of the board and through an annular space between the wall of the through hole and the lead inserted to the through hole from the upside of the board by means of a capillary phenomenon with wetting, and then the solder material solidifies by drop of temperature to form a connecting portion made of the solder material. On the other hand, most of the solder material which is supplied as described above is not provided into the annular space and returns to the solder bath by the gravity while it remains in the molten state (i.e. before solidifying), and it is supplied again in the form of the solder wave.
In this way, the connecting portion made of the solder material is formed to electrically and mechanically connect the lead of the electronic component with the land formed on the board, and thereby an electronic circuit board is produced.
For such an electronic circuit board, a solder material of an Snxe2x80x94Pb system which includes Sn and Pb as major components has been generally used hitherto. However, it is beginning to use a so-called xe2x80x9clead-freexe2x80x9d solder material (i.e. a solder material which substantially comprises no lead element), especially an Sn based solder material on an industrial scale as the alternative to the lead-including solder material since a lead element in the Snxe2x80x94Pb based solder material may cause environmental pollution if it is wasted through wrongful disposal.
When electronic circuit boards are produced on an industrial scale continuously for a long duration by using the lead free solder material (which is also simply referred to as xe2x80x9ca solder materialxe2x80x9d hereinafter) in the flow soldering process as described above, there may arise problems of a higher defective fraction which is derived from so-called xe2x80x9cbridgesxe2x80x9d, insufficient solder wetting of the through hole and the lead (xe2x80x9chu-nurexe2x80x9d in Japanese) or the like, and of lower reliability characteristics which are derived from a so-called xe2x80x9clift-offxe2x80x9d phenomenon, a lowered connection strength or the like compared with a case using the lead-including solder material. Thus, in a case using the lead-free solder material, flow soldering is practically conducted while at least a part of the solder material in a solder bath is periodically replaced with a fresh solder material in order to avoid such problems.
We have found that the problems as described above are originated in a change of a composition of the solder material since the molten solder material is contacted with the board in the form of the wave(s), and the most of the solder material is recovered in the solder bath, followed by being recycled in the flow soldering process, which will be described in detail as below.
In the flow soldering process, the board which is contacted with the molten solder material in the form of the wave includes various members, and materials used for some of those members (e.g. a wiring pattern formed on the underside of the board, a plating metal and a base metal of a lead of an electric component) may elute into the solder material as contaminants or impurities (or additional components) of the solder material when the members contact with the molten solder material. An Snxe2x80x94Pb based material is still prevailingly used as a plating material for a lead of an electronic component currently, so that it is inevitable for Pb to accumulate as a contaminant in the solder material. In addition, an Snxe2x80x94Bi based material is also used as a lead-free plating material in recent years, and Bi possibly melts and accumulates as a contaminant in the solder material in this case. Furthermore, a Zn-containing alloy material is used as a base material for a lead, and Zn possibly melts and accumulates as a contaminant in the solder material if a plating material covering such base material melts. In addition to these elements, Cu which is used as a wiring pattern material may become a contaminant (or an additional component). A part of the solder material supplied in the form of the wave which has not been used for connection with the electronic component is returned to and recovered in the solder bath while containing such contaminants. Thus recovered solder material is again supplied in the form of the wave(s). By repeating the above supplying and returning, the contaminants accumulate gradually in the molten solder material located in the solder bath, so that a composition of the solder material shifts from an initial composition.
It has been found that such shift of composition may cause the problems of the higher defective fraction of products and the lower reliability characteristics described above in the case using the lead-free solder material, although such shift hardly causes any problem in the case using the Snxe2x80x94Pb based solder material. For example, from results of tensile tests as acceleration tests of a solder material which is prepared by adding Pb as an impurity to a lead-free solder material, it has been found that a destructive mode of a connection changes from that of a connection made by the original lead-free solder material. Moreover, according to results of tests as to lift-off occurrence rate of a solder material which is prepared by adding Cu or Bi to an Snxe2x80x94Cu based solder material (e.g. a material which consists substantially of about 0.7% by weight of Cu and the balance of Sn), it has been found that the lift-off generation rate rises with the increase in an amount of the added Cu or Bi. Moreover, it has turned out that a rate of defective products having bridges rapidly increases when a content of Cu exceeds a certain point in the Snxe2x80x94Cu based solder material.
Therefore, in order to prevent the fact that the higher defective fraction and the lower reliability characteristics are brought about remarkably, it is necessary to check (or analyze) a composition of the solder material in the solder bath periodically, and to control the composition. In order to check whether the composition of the solder material is maintained appropriately or not, it is generally conceivable to obtain a sample of the solder material from the solder bath and to analyze this sample by means of an analyzing instrument such as an atomic absorption spectroscopy or an X ray analysis instrument. However, such way of analysis has other problems in that these instruments requires complicated operations and take a long period, for example several days, although it can perform analysis of the composition with a high accuracy.
Then, the present invention has been made in order to realize that a quality of a lead-free solder material used for a flow soldering process can be estimated conveniently.
We have found that a quality of a lead-free solder material used for a flow soldering process can be estimated with ease based on a differential thermal analysis curve (or a differential thermal analysis thermogram) which is obtained as to the lead-free solder material, and have completed the present invention.
According to the present invention, there is provided a method for estimating a quality of a lead-free solder material which is used for a flow soldering process, which method comprises obtaining a differential thermal analysis curve of a sample of the lead-free solder material relatively to a reference material by utilizing a differential thermal analysis method so as to estimate the quality of the lead-free solder material based on the differential thermal analysis curve, and also provided a quality estimation apparatus for such method.
For example, it is conceivable to compare, in order to estimate the quality of the solder material, the differential thermal analysis curve of the sample of the lead-free solder material in question with other differential thermal analysis curve of a sample of a lead-free solder material having a criterial composition. As the later sample of the lead-free solder material having the criterial composition, a sample of a solder material which has, for example, an initial composition or a critical composition can be used. The comparison as described will be explained in detail later.
Alternatively, it is conceivable to compare, in order to estimate the quality of the solder material, a characteristic value derived from the differential thermal analysis curve of the sample of the lead-free solder material in question with other characteristic value as a threshold value derived from other differential thermal analysis curve of a sample of a lead-free solder material having a criterial composition, preferably a critical composition which is obtained according to a predetermined manner. For example, the predetermined manner is selecting a peak value, a liquidus point or a solidus point of the differential thermal analysis curve of each sample as the characteristic value. This comparison as described will be explained in detail later.
We have found that it is possible to estimate whether a solder material has a composition shifted from a predetermined composition or not, i.e. to estimate the quality of the solder material, based on the differential thermal analysis curve obtained as to the solder material, which will be described below in detail. According to the quality estimation method and/or the quality estimation apparatus of the present invention, it is possible to estimate the quality of the solder material by a simple operation in a short period, and therefore the quality of the solder material can be estimated by an ordinary operator immediately. Moreover, the quality estimation apparatus of the present invention has advantages in that it can be installed in a smaller space because of its compact size and can minimize an influence of a production cost of an electronic circuit board because of its lower price even though the quality estimation is conducted routinely, when compared with a conventional analyzing instruments.
Furthermore, according to the present invention, there is also provided a flow soldering process and a flow soldering system in which the quality estimation method and/or the quality estimation apparatus of the present invention is utilized. In addition to the advantages similar to those of the quality estimation method and/or the quality estimation apparatus of the present invention, the flow soldering process and the flow soldering system of the present invention have advantages in that it can estimate the quality of the solder material with ease at a site where the flow soldering is carried out, and that the quality of the solder material can be controlled routinely.